Many wellbore operations require cutting of metallic objects, such as tubing, casing, drill pipe or coiled tubing, in order to release the objects and any associated tools for removal from the wellbore. For example, when conducting drilling operations, it is not uncommon for a drill bit to become stuck. In such a situation, it may be desirable to cut the drill pipe at a location above the drill bit, such that the drill pipe can be retrieved, the drill bit fixed, and drilling operations can be resumed. Cutting efficiency and the necessity of salvaging equipment in close proximity to the drill bit (such as steering equipment, logging equipment, sensors, and other tools) may result in a desire to make the cut at a precise location along the drill string, such as at a joint between two sections of pipe in the drill string or even at a particular thread location in such a joint.
This type of precision may also be necessary for other downhole cutting activities. For example, a cut-to-release packer may provide a window of only a few inches within which a circumferential cut must be made in order to retract the packer's slips and retrieve the packer from the wellbore. Similarly, certain operations may require multiple cuts that must be made at the same location on different trips. Other downhole cutting and non-cutting operations require similar precision in tool placement.
In addition, even when a downhole tool can be placed at a desired location, it is often difficult to maintain the position for the duration of the operation. For example, cutting torches that produce a high pressure jet of gases during operation often create a fluid imbalance that results in the axial movement of the tool and an undesirable cut. To overcome these challenges, it is often necessary to perform a pre-cut operation to allow for fluid balancing between the drill string and the annulus. This requires a separate trip into the wellbore for the pre-cut operation prior to the necessary cutting operation.
While the tools required for these operations can be lowered into the wellbore from the surface using a measurable length of slickline, wireline, coiled tubing, or pipe, there are often difficulties in determining the precise location of the tool due to the elasticity of the lowering material. A small degree of elasticity (which is often an unknown parameter) may result in an unacceptably large error in calculated depth at the depths at which many of these operations take place. Such errors are exacerbated in deviated wells. Accordingly, it is difficult to know the location of a downhole tool with the precision that is required. Existing solutions, such as no-go shoulders, function by intentionally creating an undesirable restriction in the downhole conduit. Moreover, existing solutions do not address the problem of maintaining a downhole tool in the desired location throughout the duration of the operation.
There is therefore a need for methods and apparatus to position a downhole tool with a high degree of precision and to maintain the location of the tool throughout a downhole operation.